RF power amplifier

ABSTRACT

An RF power amplifier according to the invention comprises a plurality of parallel output transistors (HBT, 1,1  to HBT, 1 ,N) connected to a power supply. A plurality of base resistors (Rb, 1,1  to Rb, 1 ,N) for the output transistors (HBT, 1,1  to HBT, 1 ,N) and a plurality of input capacitors (Cb, 1  to Cb,N), each coupled in parallel to receive an RF signal input and connected via at least one additional passive component to the inputs of each corresponding output transistor (HBT, 1,1  to HBT, 1 ,N), are provided An output for an RF output signal is obtained from the parallel connection of the output transistors (HBT, 1,1  to HBT, 1 ,N). The transistors (HBT, 1,1  to HBT, 1 ,N) are heterojunction bipolar transistors.

The invention relates to a radio frequency power amplifier comprising aplurality of amplifying elements each with a control electrode, a firstmain electrode, and a second main electrode, whereby the first mainelectrodes of the amplifying elements are coupled to a common outputnode, the second main electrodes of the amplifying elements are coupledto a common reference node, the control electrode of each of theamplifying elements is coupled to a common radio frequency signal inputnode via a respective input capacitor and to a common bias node via arespective base resistor.

U.S. Pat. No. 5,629,648 discloses a radio frequency (RF) power amplifiercircuit comprising heterojunction bipolar transistors and including afirst transistor having a base-emitter voltage; a power supply and apower supply resistor connected to the power supply, causing a DCcurrent to flow through the first transistor which develops a resultantvoltage equal to the base-emitter voltage of the first transistor. Atleast two manifold base resistors and at least two output transistorsare provided, each of which receives the resultant voltage through itscorresponding manifold base resistor. At least two input capacitors,each coupled in parallel to receive the RF signal input and connected tothe input of each corresponding output transistor, have a common inputconnected to the RF signal input and have individual outputs that are DCisolated from each other and are connected to each output transistor. AnRF output signal is obtained from the parallel connection of the outputtransistors, provided that each transistor is connected to ground.

Heterojunction bipolar transistors are suitable for radio frequency (RF)power amplifier applications. Generally speaking, a lot of power isdissipated in a power device. In order to spread the heat sufficiently,thus avoiding constructive breakdown, such power amplifiers aretypically made by distributing many small output transistor cells evenlyover a large semiconductor area. All cells are fabricated on a piece ofsemiconductor material (Si or GaAs) and are connected in such a way thatthey operate in parallel. In this way many small devices can generate ahigh power level. Two well known problems of such amplifiers are theirelectrical stability and linearity.

It is an object of the invention to provide a transistor RF poweramplifier having an improved stability.

To this end the invention provides a radio frequency power amplifier asdefined in the opening paragraph which is characterized in that thecontrol electrode of each of the amplifying elements is coupled to saidrespective capacitor (Cb,1 . . . Cb,N) and to said respective baseresistor (Rb,1,1 . . . Rb,1,N) via a respective additional passivecomponent (Rb,2,1 . . . Rb,2,N).

A further advantage of the RF power amplifier according to the inventionis an improved linearity.

Additional advantageous features of the radio frequency power amplifieraccording to the invention are claimed in the dependent claims.

These and various other advantages and features of novelty whichcharacterize the present invention are disclosed in detail in theappended claims. However, for a better understanding of the invention,its advantages, and the object achieved by its use, reference is made tothe drawings which also form part hereof, and to the accompanyingdescription, illustrating preferred embodiments of the presentinvention, with reference to the drawings, in which:

FIG. 1 shows an RF power amplifier circuit according to an embodiment ofthe invention;

FIG. 2 shows a graph demonstrating the positive effect of the additionalresistor on the stability of the RF power amplifier circuit of FIG. 1;

FIG. 3 shows a Smith chart with stability circles, also demonstratingthe positive effect of the additional resistor on the stability of theRF power amplifier circuit of FIG. 1;

FIG. 4 shows the effect of the value of Rb,2 on the electrical gain andpower added efficiency at the 900 MHz frequency band; and

FIG. 5 shows the effect of the value of Rb,2 on the electrical gain andpower added efficiency at the 1800 MHz frequency band.

FIG. 1 shows an RF power amplifier circuit comprising heterojunctionbipolar output transistors HBT,1,1 to HBT,1,N, the base terminals ofwhich are connected to base resistors Rb,1,1 to Rb,1,N which in turn areconnected to the power supply at a Bias terminal. Input capacitors Cb,1to Cb,N are each coupled in parallel to receive the RF signal input andare connected, via additional resistors Rb,2,1 to Rb,2,N to theindividual input of each corresponding output transistor HBT,1,1 toHBT,1,N. The input capacitors Cb,1 to Cb,N have one terminal connectedto the RF signal input terminal Rfin. The other terminals of the inputcapacitors Cb,1 to Cb,N are DC isolated from each other and areconnected, via the additional resistors Rb,2,1 to Rb,2,N, to theindividual output transistors HBT,1,1 to HBT,1,N. An RF output signal isobtained from the parallel connection of the output transistors HBT,1,1to HBT,1,N.

The addition of the resistors Rb,2,1 to Rb,2,N has a positive effect onthe stability of the whole circuit. In order to calculate the effect onthe stability, the so-called stability K-factor is calculated, asindicated schematically in FIG. 2, for a heterojunction bipolartransistor RF power amplifier consisting of 24 cells (N=24), each cellhaving an Rb,1 an Rb,2 and a Cb.

In order to assess the influence of the additional resistor onstability, the following simulation was carried out. The ratio ofRb,2/Rb,1+Rb,2 was varied while keeping the sum Rb,1+Rb,2 constant. Thesum Rb,1+Rb,2 is kept constant in order to maintain the same temperaturestability. FIG. 2 shows a graph of the K-factor versus frequency, wherethe parameter x was varied from 0 to 0.05, the intermediate values beingx=0.01, x=0.02, x=0.03, and x=0.04. The effect of the resistor Rb,2 onthe stability is thus demonstrated.

When increasing the value of Rb,2, the frequency at which the K-factoris 1 drops. If the K-factor is above 1, the device is stable. So byadding Rb,2 and by increasing its value, the device becomes more stable.This is demonstrated by the stability circles of FIG. 3 which shows aSmith chart with stability circles showing the benefit of adding theresistor Rb,2. In connection with the load stability circles, theparameter x was varied from 0 to 0.05, the intermediate values beingx=0.01, x=0.02, x=0.03, and x=0.04. In connection with the sourcestability circles, the parameter x was varied from 0 to 0.05, theintermediate values being x=0.01, x=0.02, x=0.03, and x=0.04. Increasingthe value of Rb,2 makes the stability circles smaller, indicating thatthe device becomes less sensitive to input impedance variations whichhave an adverse effect on the stability.

The addition of the resistor Rb,2 in the circuit has an effect on otherelectrical parameters, most noticeably the electrical gain and the poweradded efficiency. In order to estimate a good trade-off between improvedstability and linearity the gain and power added efficiency wascalculated as a function of the value of Rb,2 with respect to Rb,1. Thissimulation was done for two power devices that can generate power levelsthat are typically used in GSM and DCS mobile phones.

FIGS. 4 and 5 show the effect of the value of Rb,2 on the electricalgain and power added efficiency in the 900 MHz and the 1800 MHzfrequency bands. It is apparent from these Figures that the effect onthe electrical gain and power added efficiency is low if the value ofthe resistor Rb,2 is low. Hence, the addition of the resistor Rb,2offers an improvement as regards stability and allows an additionaldegree of freedom in designing an RF power amplifier usingheterojunction bipolar transistor technology.

Further improvements to optimize the trade-off between stability andlinearity on the one hand and electrical gain and power added efficiencyon the other hand can be achieved by adding additional passive resistivecomponents in parallel and/or in series with resistor Rb,2 or by addingadditional passive capacitive components in parallel with resistor Rb,2.

The embodiments of the present invention described herein are intendedto be taken in an illustrative and not a limiting sense. Variousmodifications may be made to these embodiments by persons skilled in theart without departing from the scope of the present invention as definedin the appended claims.

1. A radio frequency power amplifier comprising a plurality of bipolaramplifying elements each with a control electrode, a first mainelectrode, and a second main electrode, whereby the first mainelectrodes of the amplifying elements are coupled to a common outputnode, the second main electrodes of the amplifying elements are coupledto a common reference node, the control electrode of each of theamplifying elements is coupled to a common radio frequency signal inputnode via a respective input capacitor and to a common bias node via arespective base resistor, characterized in that the control electrode ofeach of the amplifying elements is coupled to said respective capacitorand to said respective base resistor via a respective additional passivecomponent.
 2. A radio frequency power amplifier comprising a pluralityof amplifying elements each with a control electrode, a first mainelectrode, and a second main electrode, whereby the first mainelectrodes of the amplifying elements are coupled to a common outputnode, the second main electrodes of the amplifying elements are coupledto a common reference node, the control electrode of each of theamplifying elements is coupled to a common radio frequency signal inputnode via a respective input capacitor and to a common bias node via arespective base resistor, characterized in that the control electrode ofeach of the amplifying elements is coupled to said respective capacitorand to said respective base resistor via a respective additional passivecomponent, characterized in that the additional passive component is aresistor.
 3. A radio frequency power amplifier as claimed in claim 2,characterized in that the additional resistor has an impedance that issmall with respect to the impedance of the corresponding base resistor.4. A radio frequency power amplifier as claimed in claim 2,characterized in that further additional passive components are providedin parallel and/or in series with the additional resistor.
 5. A radiofrequency power amplifier, comprising a plurality of amplifying elementseach with a control electrode, a first main electrode, and a second mainelectrode, whereby the first main electrodes of the amplifying elementsare coupled to a common output node, the second main electrodes of theamplifying elements are coupled to a common reference node, the controlelectrode of each of the amplifying elements is coupled to a commonradio frequency signal input node via a respective input capacitor andto a common bias node via a respective base resistor, characterized inthat the control electrode of each of the amplifying elements is coupledto said respective capacitor and to said respective base resistor via arespective additional passive component, characterized in that theamplifying elements are heterojunction bipolar transistors.